1. Field of the Invention
The invention relates to photonic band gap materials, particularly techniques for forming such materials.
2. Discussion of the Related Art
Recently, there has been increasing interest in periodic dielectric structures, also referred to as photonic crystals (PC) for numerous photonics applications. Of particular interest are photonic crystals exhibiting gaps in photonic band structures, referred to as photonic band gap (PBG) materials. See, e.g., P. S. J. Russell, xe2x80x9cPhotonic Band Gaps,xe2x80x9d Physics World, 37, August 1992; I. Amato, xe2x80x9cDesigning Crystals That Say No to Photons,xe2x80x9d Science, Vol. 255, 1512 (1993); and U.S. Pat. Nos. 5,600,483 and 5,172,267. PBG materials exhibit a photonic band gap, analogous to a semiconductor""s electronic band gap, that suppresses propagation of certain frequencies of light, thereby offering, for example, photon localization or inhibition of spontaneous emissions.
PBG materials are generally formed by providing a high refractive index dielectric material with a three-dimensional lattice of cavities or voids having low refractive index. Electromagnetic radiation tends to localize within the high dielectric material to achieve a lower overall energy. As the feature size of the material approaches the wavelength of the radiation, this localization is inhibited by the finite region of space which the field must occupy, and the field must therefore spread over both the high and low dielectric regions. The energy of the radiation, and thus the frequency, varies accordingly with the distribution of the field in the high and low dielectric regions. Thus, for systems which exhibit a spatially varying dielectric profile, the energy of the field tends to depend strongly on the wavelength of the radiation and on the direction of propagation. For periodic systems, this variation in energies can be portrayed in a band structure diagram. And because the structures of interest typically show a gap in energies in such a band structure, they have been deemed photonic bandgap materials.
The concept and theory of PBG materials has been extensively studied, but experimental realization of such theory has proven to be difficult. In particular, it has been difficult to organize a three-dimensional lattice with periodicities of a micron scale, i.e., for optical and near-infrared frequencies. (Periodicities on a micron scale, as used herein, indicate that a structure contains repeating units, the repetition occurring at a distance falling within the range 0.1 xcexcm to 100 xcexcm.) Materials of larger periodicities, e.g., centimeter and millimeter scale, which are suitable for microwave applications, have been easier to prepare.
In one early approach to PC formation, reflected in the above-cited U.S. Patents, solid materials are provided with numerous holes by a mechanical technique such as drilling. This approach has provided useful results, but is limited by the ability of current processing technology to provide the necessary structure. Drilling, for example, is not capable of providing periodicity on a micron scale.
In another approach, ordered colloidal suspensions or sediments of relative low refractive index particles such as silicon dioxide or polystyrene, referred to as colloidal crystals, are used as templates for infiltration or deposition of high refractive index materials in a desired structure, and the particles are then etched away or burned out to provide the voids. See, e.g., B. T. Holland et al., xe2x80x9cSynthesis of Macroporous Minerals with Highly Ordered Three-Dimensional Arrays of Spheroidal Voids,xe2x80x9d Science, Vol. 281, 538 (July 1998); E. G. Judith et al., xe2x80x9cPreparation of Photonic Crystals Made of Air Spheres in Titania,xe2x80x9d Science, Vol. 281, 802 (July 1998); and A. A. Zakhidov et al., xe2x80x9cCarbon Structures with Three-Dimensional Periodicity at Optical Wavelengths,xe2x80x9d Science, Vol. 282, 897 (October 1998). The infiltration/deposition has been performed, for example, by an alkoxide sol-gel technique and by chemical vapor deposition. The results attained thus far have been interesting, but have not yet proven a commercially feasible process.
In seeking techniques for providing periodicities on a micron scale, lithography of silicon has become a prime candidate. Specifically, silicon has a high dielectric constant (12.1 at 1.55 xcexcm) and is transparent in the infrared regime. Also, extensive work in patterning silicon on a micron scale has already been done for microelectronic applications. Unfortunately, extending the two-dimensional knowledge from such microelectronics applications to the three-dimensional periodic structures necessary for PBG materials has proven to be difficult. One group has published an approach for the initial stages of preparing a logpile structure in silicon (S-Y Lin and J. G. Fleming, xe2x80x9cA Three-Dimensional Optical Photonic Crystal,xe2x80x9d Journal of Lightwave Technology, Vol. 17, No. 11, 1944 (1999).) According to this approach, formation of silicon bars 180 nm wide is required to achieve a bandgap at a desired wavelength of 1.55 xcexcm. The group, however, has only reported results for a five-layer stack, which is believed to be too thin to exhibit a complete band gap. And the small width of the silicon bars substantially increases the difficulty and complexity of forming a sufficient three-dimensional structure.
Another group has proposed a structure theoretically capable of being formed through a series of lithographic steps in silicon. (U.S. Pat. Nos. 5,440,421 and 5,600,483 to Fan et al.) However, the structure is exceedingly complex, and realization of the structure would be extremely difficult given even the best available lithographic techniques. In particular, their approach requires etching narrow holes with very high aspect ratios through dissimilar materials, at the conclusion of the fabrication process.
It would therefore be desirable to develop processes for fabrication of PBG materials having periodicities on a micron scale, where such processes are more readily performed than currently proposed techniques.
The invention provides a improved lithographic process for fabricating articles comprising photonic band gap materials with micron-scale periodicities. Significantly, and in contrast to other proposed techniques, the approach of the invention is able to be readily performed by current lithographic processes and equipment.
In general terms, the process of the invention involves providing a three-dimensional structure made up of a plurality of stacked layers, where each layer contains a substantially planar lattice comprising shapes of a first material, typically silicon, with interstices between the shapes. Each shape contacts at least one shape of an adjacent layer, the interstices throughout the plurality of layers are interconnected, and the interstices comprise a second material, e.g., silicon dioxide. Typically, the second material is etched from the interconnected interstices to provide a structure of the first material and air. The shapes are generally extruded shapes, e.g., cylinders or squares, which tend to ease the lithographic process. (Optionally, the lattices further comprise one or more engineered defects, or portions thereof, including point defects, line defects, plane defects, and other defects that provide desired properties.)
In one aspect (reflected in FIGS. 1A to 1H), the process involves forming a silicon layer 14 on a substrate 10 (the layer formed in an x-y plane), etching a lattice of extruded shapes 16, e.g., cylinders, in the silicon layer, forming a silica on dioxide layer 18 on the lattice, and planarizing the silicon dioxide layer down to the top surfaces of the extruded shapes (see FIG. 1D). These steps are then repeated for, e.g., second, third, fourth, and fifth silicon layers. However, as reflected in FIGS. 1F-1H, the (x,y) location of each lattice is shifted from the (x,y) location of the immediately preceding lattice by a distance, d, in a direction along the x or y axis according to the repeating sequence x, y, xe2x88x92x, xe2x88x92y.
For example, the second lattice is shifted a distance d in the x direction relative to the (x,y) location of the first lattice, the third lattice is shifted the same distance d in the y direction relative to the (x,y) location of the second lattice, the fourth lattice is shifted the distance d in the xe2x88x92x direction relative to the (x,y) location of the third lattice, and the fifth lattice is shifted the distance d in the xe2x88x92y direction relative to the (x,y) location of the fourth lattice. This sequence puts the fifth lattice directly over the first lattice, i.e., at the same x-y coordinates (see FIG. 1H). Once the desired number of lattices are formed, the silicon dioxide is etched to provide a silicon/air structure, as shown in FIG. 2. (In addition to the above technique, is it possible to first deposit a layer of the second material, e.g., silicon dioxide, etch out the lattice of shapes from the second material, and then deposit a layer of the first material, e.g., silicon, into the etched-out shapes.)
In one particular design, the extruded shapes are silicon cylinders with a diameter of about 390 nm and a height of about 210 nm, the columns within each lattice have a pitch of 600 nm, and d is 300 nm. In this configuration, the structure is calculated to exhibit a three-dimensional photonic bandgap centered at about 1.55 xcexcm and extending from about 1.45 to about 1.65 xcexcm, which completely covers the commonly-used optical communications band of 1.5 to 1.6 xcexcm.